Because of the problems associated with traditional antibody technology such as obtaining antibody from sera or hybridoma technology, genetic engineering has been used with increasing frequency to design, manipulate, and produce antibodies or antibody derivative molecules (such as bispecific fusion proteins) with a desired set of binding properties and effector functions.
Difficulties encountered with the production of stable hybridomas producing human antibody have led to the development of alternative technologies designed to circumvent in vivo antibody production and conventional in vitro techniques (Mayforth R. D., Quintans, J. (1990) Current Concepts: Designer and catalytic antibodies. New Eng. J. Med. 323:173-178; Waldmann, T. A. (1991) Monoclonal antibodies in diagnosis and therapy. Science 252:1657-1662; Winter, G., Milstein, C. (1991) Man-made Antibodies. Nature 349:293-299; Morrison, S. L. (1992) In Vitro antibodies: strategies for production and application. Ann. Rev. Immunol. 10:239-266).
Initial attempts to couple the binding specificities of two whole antibodies against different target antigens for therapeutic purposes utilized chemically conjugated "heteroconjugate" molecules (Staerz, U. D., Kanagawa, O., Bevan, M. J. (1985) Hybrid antibodies can target sites for attack by T cells. Nature 314:628-631; Perez, P., Hoffman, R. W., Shaw, S., Bluestone, J. A., Segal, D. M. (1985) Specific targeting of cytotoxic T cells by anti-T3 linked to anti-target antibody. Nature 316:354-356; Liu, M. A., Kranz, D. M., Kurnick, J. T., Boyle, L. A., Levy, R., Eisen, H. N. (1985) Heteroantibody duplexes target cells for lysis by cytotoxic T lymphocytes. Proc. Natl. Acad. Sci. USA 82:8648-8652; Jung, G., Ledbetter, J. A., Muller-Eberhard, H. J. (1987) Induction of cytotoxicity in resting human T lymphocytes bound to tumor cells by antibody heteroconjugates. Proc. Natl. Acad. Sci. USA 84:4611-4615; Emmrich, F., Rieber, P., Kurrie, R., Eichmann, K. (1988) Selective stimulation of human T lymphocyte subsets by heteroconjugates of antibodies to the T cell receptor and to subset-specific differentiation antigens. Eur. J. Immunol. 18:645-648; Ledbetter, J. A., June, C. H., Rabinovitch, P. S., Grossmann, A., Tsu, T. T., Imboden, J. B. (1988) Signal transduction through CD4 proximity to the CD3/T cell receptor. Eur. J. Immunol. 18:525-532).
These attempts demonstrated that monoclonal antibodies directed against the murine or human CD3 T cell surface receptor chemically linked to anti-target cell antibodies trigger lysis of target cells by cytotoxic T lymphocytes (CTL), overcoming the major histocompatibility complex restriction of CTL.
Bispecific antibodies have been produced from hybrid hybridomas by heterohybridoma techniques and have demonstrated properties in vitro similar to those observed for heteroconjugates (Milstein, C., Cuello, A. C. (1983) Hybrid hybridomas and their use in immunohistochemistry. Nature 305:537-540; Staerz, U. D., Bevan, M. J. (1986) Hybrid hybridoma producing a bispecific monoclonal antibody that can focus effector cell activity. Proc. Natl. Acad. Sci. USA 83:1453-1457; Clark, M. R., Waldmann, H. (1987) T-cell killing of target cells induced by hybrid antibodies: comparison of two bispecific monoclonal antibodies. J. Natl. Cancer Inst. 79:1393-1401; Lanzavecchia, A., Scheidegger, D. (1987) The use of hybrid hybridomas to target cytotoxic T lymphocytes. Eur. J. Immunol. 17:105-111; Gilliland, L. K., Clark, M. R., Waldmann, H. (1988) Universal bispecific antibody for targeting tumour cells for destruction by cytotoxic T cells. Proc. Natl. Acad. Sci. USA 85:7719-7723). However, such antibodies were produced from cell fusions.
Despite the promising results obtained using heteroconjugates or bispecific antibodies from cell fusions, several factors made them impractical for large scale therapeutic applications. Such factors include (1) rapid clearance of large heteroconjugates in vivo, (2) the labor intensive techniques required for generating either type of molecule, (3) the need for extensive purification away from the homoconjugates or mono-specific antibodies, and (4) low yields.
Generally, procedures associated with using heteroconjugates or bispecific antibodies involve co-expression approaches with two different specificities in which the sequences encoding the heavy (H) and/or light (L) immunoglobulin chains are not linked and thus suffer from the problem of random H-L association, and/or random (HL)-(HL) association, leading to only a small percentage of correct product and to difficult purification schemes. Purification may become cumbersome and the characterization difficult, if there is an excessive number of monospecific or non-specific protein molecules.
In an effort to eliminate these problems, genetic engineering has been used to generate bispecific or bifunctional single chain antibodies in vitro (Haber et al., 1990; Wels, W., Harwerth, I. M., Zwickl, M., Hardman, N., Groner B., Hynes, N. E. (1992) Construction, bacterial expression and characterization of a bifunctional single-chain antibody phosphatase fusion protein targeted to the human ERBB-2 receptor. Biotechnology 10:1128-1132; A. Traunecker et al. (1991) EMBO Journal 10(12):3655-3659). However, such efforts have not been promising.
Bispecific or bifunctional single chain antibodies have been produced in a bacterial system. However, such fusion proteins have been produced in inactive form (Haber et al., 1990). Further, the fusion proteins so produced exhibit reductions in binding affinities and/or avidities or require complicated isolation and purification procedures to recover the desired products (Haber et al., 1990; Wels et al., 1992a).
Monovalent single chain antibodies and bifunctional single chain antibodies have been expressed (Wels, 1992b). The antibody molecules were genetically engineered to minimize their size and to allow for their functional modification. Moreover, the bifunctional antibody is bifunctional only in that the bacterial alkaline phosphatase gene was joined 3' to the scF.sub.v gene. These bifunctional antibodies include a single binding domain (e.g., V.sub.L +V.sub.H) and the alkaline phosphatase gene was used merely as a marker to detect the antibodies so bound to its target.
Janusin molecules containing FvCD3 and CD4 sequences have been expressed (A. Traunecker et al. "Bispecific single chain molecules (Janusins) target cytotoxic lymphocytes on HIV infected cells, EMBO Journal 10(12):3655-3659). The janusin construct comprises a portion of the CD4 molecule in the amino terminus of the construct and the binding domain (i.e., V.sub.L +V.sub.H) of CD3 in the carboxy terminus of the construct. Janusin molecules do not comprise helical peptide linkers which separate the CD3 variable regions from portions of the CD4 molecule. Moreover, janusin molecules are sometimes found in multimeric or aggregate forms. Additional purification is sometimes required to avoid aggregate formation.
There is a need for the subject invention in view of the problems discussed hereinabove concerning antibody production. At present, there is a persisting problem associated with antibody technology, namely, the difficulty in obtaining large quantities of specific antibody. Historically, antibodies were obtained from sera or hybridomas of mouse origin. However, sera were often of limited quantity and variable quality. Moreover, antibodies of mouse origin have limited usefulness for human treatment because of their propensity to initiate an immune response sometimes deleterious to non-mouse subjects.
In order to overcome the problems which specifically plague antibody technology and more generally the problems associated with the production of substantial amounts of functional protein molecules, a new expression vector which facilitates the expression of biologically active fusion proteins is described herein.